1. Field of the Invention
The present invention relates to a method for fabricating a welded structure in which square sections of aluminum or an aluminum alloy, (hereinafter referred generically to aluminum material") are welded to form a body frame of an automobile or the like. Particularly, the invention is concerned with a welded structure having high reliability and strength of welded portions.
2. Description of the Related Art
Heretofore, a space frame structure has generally been used for supporting the body of an automobile or a transport plane through hollow pipe frames. FIG. 17 is a schematic diagram showing a space frame structure which is applied to an ordinary type of passenger cars. As shown in the same figure, a plurality of sections 51 which are each in the shape of a square pipe are provided and are welded at end portions thereof to end portions of other sections 51 to form welded portions 52. The sections 51 are fixed together by the welded portions 52 to form a space frame structure. An automobile body 54 is placed so as to cover the space frame 53. Various concrete methods have been proposed for forming the welded portions 52.
FIGS. 18 and 19 are perspective views each showing a welded portion 52 on a larger scale. In FIG. 18, an end portion 56a of a section 56 is in abutment with a side wall of an end portion 55a of a section 55, and the section 56 is fixed to the section 55 by forming a weld bead around the abutted portion. This welding method is generally adopted for the fabrication of a space frame because in comparison with other welding methods it is easy to weld the end portion 56a of the section 56 to the side wall of the end portion 55a of the section 55. There also is known such a welding method as shown in FIG. 19, which method uses a connector member 58. The connector member 58, which is bent 90.degree. at the central portion, is disposed between the sections 55 and 56, and the end portion 55a of the section 55 is brought into contact with one end of the connector member 58, while the end portion 56a of the section 56 is contacted with the other end of the connector member 58. Then, weld beads 57a and 57b are formed around the contacted portions to connect the sections 55 and 56 through the connector member 58.
Various connector shapes and various connector-section connecting methods have been proposed (Japanese Patent Laid Open Nos. 80570/95 and 137655/95). However, the omission of connectors is desired because the use of connectors not only results in increase in weight of the vehicle body frame but also results in increase of the manufacturing cost. Therefore, a welded structure fabricated by direct connection of sections without the use of any connector member and capable of enhancing the weld strength has been proposed (Japanese Patent Publication No. 59838/94).
FIG. 20 is a perspective view showing welded structures (prior art). As shown in FIG. 20(a), there is used a section 61 which is in the shape of a square pipe. Of faces of the section 61, the face opposed to a section 62 is a main wall 61a having a thickness several times as large as the thickness of other walls 61b. An end portion of the section 62 is in abutment with the main wall 61a, and fillet welding is applied to this abutted portion to form a weld bead 57.
In the welded structure constructed as above, the main wall 61a is thicker than the other walls 61b, so when a load indicated by arrows A is imposed on the welded structure, the magnitude of a strain induced at a central portion 62a and that of a strain induced at an end portion 62b become almost equal to each other. That is, in the case where the main wall and the other walls are of the same thickness, the strain of the end portion 62b is larger than that of the central portion 62a, so that breaking may start from that strain concentrated portion. In the welded structure shown in FIG. 20(a), such breaking is prevented and both fatigue strength and rigidity are improved because of a uniform magnitude of strain. As shown in FIG. 20(b), the inner surface of the main wall 61a may be centrally formed at the central portion with a projecting portion 61c instead of thickening the main wall 61a.
However, the aforementioned prior art involves the following problems. FIG. 21 is an enlarged sectional view taken on line A--A in FIG. 20(a), and FIG. 22 illustrates in what manner the welded structure of FIG. 20(a) is formed by welding, in which (a) is a perspective view and (b) is a sectional view. As shown in FIG. 21, the angle between adjacent side walls is 90. Therefore, when fillet welding is performed along welding arrows 65, as shown in FIG. 22, it is necessary that the welding position of a torch 64 be changed abruptly at a corner 63e, and thus it is difficult to effect uniform welding.
As shown in FIG. 21, moreover, even when side walls 63a to 63d of the section 62 are equal in thickness t.sub.1, a diagonal width t.sub.2 of the corner 63e is .sqroot.2.times.t.sub.1 because the angle between adjacent side walls is 90.degree.. FIGS. 23(a) and (b) are enlarged sectional views taken on lines B--B and C--C, respectively, in FIG. 22(b), and FIG. 23(c) is a sectional view showing in what manner the weld metal portion illustrated in (b) breaks. When the welding current is set so as to give good penetration of a weld metal portion 66 of the side wall 63d, as shown in FIG. 23(a), since the thickness of the corner 63e is .sqroot.2 times the thickness of the side wall 63d, as shown in FIG. 23(b), the penetration of the weld metal portion 66 at the corner 63e becomes insufficient, thus causing a large unmelted portion to remain inside the corner 63e. The presence of such an unmelted portion poses the problem that, as shown in FIG. 23(c), when a load is given in the arrowed direction in the same figure, breaking starts from the unmelted portion, leading eventually to breaking of the weld metal portion 66a. Generally, as to a broken state of a welded structure, it is preferably caused by a plastic deformation of a welding heat affected portion and of a base metal. Such breaking of the weld metal portion as mentioned above is not preferable. Further, in the case where the sections 61 and 62 are of the same external dimensions, there arise the following problems.
FIG. 24 is a schematic diagram of a welded structure using sections 61 and 62 of the same external dimensions, and FIGS. 25(a) and (b) are sectional views taken on lines D--D and E--E, respectively, in FIG. 24. In FIGS. 24 and 25, the portions common to FIG. 20 are indicated by the same reference numerals as in FIG. 20 and detailed descriptions thereof are here omitted. Where both sections 61 and 62 are of the same external dimensions, as shown in FIG. 24, there arises the problem that the occurrence of an unwelded area becomes conspicuous in a weld bead portion 57c of the weld bead 57 which portion 57c is formed downward from the upper end of the section 61. In the position where the weld bead 57c is formed, as shown in FIG. 25(b), the thickness on the section 61 side is X, while the wall thickness of the section 62 is t, and thus there is a marked difference between the thickness X and the wall thickness, t. Consequently, although penetration on the section 62 side becomes deep, penetration on the section 61 side becomes shallow, thus giving rise to the problem that a uniform weld bead 57 is not formed and there occurs an unwelded area. Particularly, at a corner 63f, as shown in FIG. 25(a), the width in a diagonal direction is .sqroot.2 times as large, so that the proportion of the unwelded area on the section 61 side becomes still larger, in addition to that of the section 62.
The present invention has been accomplished in view of the above-mentioned problems and it is an object of the invention to provide a method for fabricating a welded structure of a high reliability having a high strength of welded portions.
According to the welded structure fabricating method in one aspect of the present invention, there are first provided first and second square sections of substantially the same wall thickness, then end portions of the first and second square sections are abutted with each other so that the angle between the end face of the first square section and an axis thereof is substantially the same as the angle between the end face of the second square section and an axis thereof, and the thus-abutted portion is welded in the circumferential direction. At the abutted portion, the width between the inner and outer peripheral edges of the end face of the first square section becomes equal to that of the end face of the second square section, so that both sections are almost equal in the degree of penetration of the respective melted portions in welding, thus affording a uniform weld bead. In this way the formation of an unmelted portion is suppressed, the reliability and strength of welded portions are improved, and breaking of weld metal portions is prevented.
Even in the above case, however, penetration in a corner of each square section is apt to be insufficient because of a large wall thickness thereof in the circumferential direction. Moreover, since it is necessary to change the welding position of a torch abruptly at the corner, it is difficult to perform welding continuously. It is preferable that the corner be roundish, whereby the wall thickness in proceeding direction of the torch in the circumferential direction is made uniform and penetration in that direction becomes more uniform, thus leading to further improvement in both reliability and strength of welded portions. Besides, it becomes possible to effect welding in a continuous manner.
At an acute part created on the abutted portion, the width between the inner and outer peripheral edges of each end face is large, so that penetration in the acute part is apt to become insufficient. To prevent this, it is preferable that the acute part be chamfered to form a plane part perpendicular to the end face of each section. As a result, the width between the inner and outer peripheral edges of the end face at the chamfered acute part becomes almost equal to the wall thickness of the abutted sections, thus resulting in uniform penetration and further improvement in strength of the welded portion. A filler metal may be applied to the plane part, whereby it is made possible to further improve the weldability.
According to the welded structure fabricating method in the second aspect of the present invention, there are first provided a square section and a cylindrical section, then an end face of the cylindrical member is chamfered so that the inner surface side thereof is convex. Any of various sectional shapes of the end face may be adopted. For example, the end face may have an inclined side portion. A through hole having a face which conforms to the end face is formed in the square section. Therefore, when the end face of the cylindrical member has an inclined side portion in its sectional shape, the through hole formed in the square section has an inclined face in its sectional shape. Next, the chamfered end face of the cylindrical section is aligned with the hole face of the square section and thereafter both are welded in the circumferential direction. As a result, the width between the inner and outer edges of the contact face of the cylindrical section and that of the square section become substantially equal to each other, and hence both reliability and strength of the welded portion are improved as in the welded structure fabricating method referred to above in the first aspect.
In forming a through hole having a hole face in a square section, no special limitation is placed on the inclination angle of the hole face, but it is preferable that the angle be 45.degree. relative to the associated side face of the square section. In this case, the side portion of the end face of the cylindrical section also has an inclination angle of 45.degree.. As a result, the contact faces of the cylindrical and square sections become equal to each other in the width between their inner and outer edges, and hence penetration in the cylindrical section and that in the square section become almost equal to each other, whereby the strength of the welded portion is further improved. Further, the cylindrical section can be stably fixed temporarily to the square section.
Also in the welded structure fabricating method according to the second aspect of the invention, it is preferable that the outer surface of each corner be roundish, as in the previous welded structure fabricating method. If the corner outer surface is not roundish, even if the width between the inner and outer edges of the contact face of the cylindrical section and that of the square section are kept equal to each other, it is difficult to make almost equal the penetration in the cylindrical section and that in the square section due to the influence of corners and side portions of the square section. Conversely, if the outer surfaces of the corners are roundish, both penetrations referred to above become almost equal to each other, so that the strength of the welded portion is further improved.
In this case, assuming that the radius of curvature of the outer surface of each corner is R and the width of a side portion of the square section is W, it is preferable that the outside diameter, d, of the cylindrical section be W-2R or less. If the outside diameter, d, exceeds W-2R, then when a through hole is formed in the square section, the outer peripheral portion of the through hole will reach the corner, resulting in deterioration in strength of the square section.
According to the welded structure fabricating method in the third aspect of the present invention, of the corners of a first square section, at least the corners opposed to a second square section has a roundish outer surface. Likewise, also as to the corners of the second square section, at least the corner opposed to the first square section has a roundish outer surface. Side portions of such sections are combined together in such a manner that their axes extended in the same direction. As a result, grooves are formed because the combined corners are roundish. Then, welding is performed along the formed grooves. Since the grooves are of a symmetric shape with side portions (boundary surfaces) of the first and second square sections as symmetric surfaces, penetration of the first square section and that of the second square section become almost equal, so that both sections are strongly combined together and the reliability of the welded structure is improved.
In this case, first and second through holes may be formed in the first and second square sections, respectively, and then both sections may be combined together so that the through holes are coaxial with each other. As a result, the first and second square sections can be positioned more easily with respect to each other. Further, the hollow spaces of the first and second square sections are contiguous to each other through the through holes, even when the event the air or any other gas present within the hollow spaces expand during welding, there is no fear of the expanded gas blowing off from the molten pool during welding, because the gas can move through the through holes.
In addition to both through holes mentioned above there may be formed a third through hole in the first square section for insertion of a fiber scope or the like therethrough to check the welding state of the sections from the inner surface side. Since the hollow portions of the first and second square sections are contiguous to each other through the first and second through holes, the fiber scope which has passed through the third through hole further passes through the first and second through holes and can enter the interior of the second square section from the first square section. In this way the welding state of the first and second square sections can be checked from the inner surface side. Even when the first and second square sections are connected to other sections, the welding state of each section can be checked from the inner surface side in the same way as above if only the hollow portions of the sections are interconnected via through holes or the like.
According to the welded structure fabricating method in the fourth aspect of the present invention, with end faces of first and second cylindrical sections in a section perpendicular to a weld bead as a reference, the first and second cylindrical sections are arranged in such a manner that the width between the inner and the outer edge of a contact face of the first section and that of the second section are substantially equal to each other, and then welding is applied around the end faces. In the case of a curved weld bead, a section perpendicular to the weld bead is assumed at each of various points of the curve. Such an arrangement of sections is obtained by ramping end portions of the sections so that the angle between one cylindrical section and an end face thereof is the same as the angle between the other cylindrical section, and thereafter bringing the end portions into abutment with each other. Alternatively, one end (end portion) of a cylindrical section (first cylindrical section) may be formed with a projection and this projection may be fitted in a recess formed in another cylindrical section (second cylindrical section). After the first and second cylindrical sections are arranged in this fashion, welding is performed around their end faces, whereby there is obtained a welded structure improved in both reliability and strength of the welded portion.
In all of the welded structure fabricating methods in the above first to third aspects, when the corners of sections used are roundish, it is preferable that the radius of curvature R of the outer surface of each corner and the radius of curvature, r, of the inner surface thereof be within the following ranges.
Radius of Curvature of Corner Outer Surface: preferably t&lt;R&lt;H.sub.1 /3
Given that an external dimension of a short side in a section perpendicular to the axis of a square section is H.sub.1 and the wall thickness thereof is t, if the radius of curvature R of the outer surface of each corner is smaller than t, it becomes difficult to make constant the wall thickness in the circumferential direction of the corner. On the other hand, if the radius of curvature R exceeds H.sub.1 /3, the sectional shape of a square section becomes generally circular, resulting in deterioration of the working efficiency. It is therefore preferable that the radius of curvature of the corner outer surface be set at t&lt;R&lt;H.sub.1 /3.
When the radius of curvature R of the corner outer surface is set in the above range, if the radius of curvature r of the corner inner surface in a section perpendicular to the axis of a square section is R-t, then the wall thickness in the circumferential direction of the corner can be made equal to the wall thickness, t. As a result, there is obtained a uniform bead and the strength of the welded portion is improved.
In the welded structure fabricating method according to the first aspect of the present invention, when a plane part perpendicular to end faces of sections is formed by chamfering the abutted portions of the sections, the width between the inner and the outer peripheral edge of the end face in each abutted portion is preferably within the following range.
Width T between Inner and Outer Peripheral Edges of End Face: preferably t/2.ltoreq.T.ltoreq.t
If the width T between the inner and outer peripheral edges of each end face is less than t/2, it is difficult to obtain a uniform weld bead because the width T is too small. In order to keep constant the width T and the wall thickness of a square section, T=t is preferred. In this case, however, the width of the plane part becomes smaller. Further, when the width T exceeds the wall thickness, t, it becomes difficult to apply a filler metal to the plane part because the width of the plane part becomes too small. Thus, when the plane part is formed, it is preferred that the width T between the inner and outer peripheral edges of the end face be set at t/2.ltoreq.T.ltoreq.t.